Chitosan is a basic polysaccharide obtained by deacetylation of chitin being present in carapace of crustacean animals such as crab and shrimp, treated with a concentrated alkali. However, because of strong hydrogen bond in molecules, chitosan itself is hardly soluble in a solvent other than an acidic aqueous solution and, even it is made into an aqueous solution, viscosity of the solution becomes extremely high whereby its handling is difficult. Therefore, in order to make chitosan into fine particles, a method where chitosan is dissolved in an acid and the resulting solution is gradually dropped into an alkaline coagulating solution as disclosed in JP-A 59-30722, JP-A 62-62827, JP-A 62-70401, JP-A 62-79201 etc., a method where a solution of chitosan or a dispersion of chitosan is mechanically treated as disclosed in JP-A 63-20302, JP-A 63-17902, JP-A 63-210101, etc., and the like are available. However, in all of those methods, only particles in a size of several tens .mu.m to several hundreds .mu.m or more are obtained and they are not dispersible in water.
Accordingly, an object of the present invention is to obtain a polymer emulsion where polymer particles of a small particle size containing chitosan are dispersed in water and also to provide use of the said polymer emulsion and of the said polymer particles therein.